Support of young growing plant tissues depends largely on the turgidity of cells restrained by an elastic cell wall comprised of three interpenetrating networks, namely, cellulosic-xyloglucan, pectin, and hydroxyproline-rich glycoproteins (HRGPs). When these networks are loosened, turgor drives cell extension. Significantly, HRGPs have no animal homologs, thus emphasizing a plant-specific function.
Quantitatively, most of the cell surface HRGPs (extensins) form a covalently cross-linked cell wall network. Unlike extensins, another set of HRGPs, arabinogalactan-proteins (AGPs) occur as monomers that are hyperglycosylated by arabinogalactan polysaccharides. AGPs are initially tethered to the plasma membrane by a lipid anchor whose cleavage results in their movement from the periplasm through the cell wall to the exterior. Although implicated in diverse aspects of plant growth and development, the precise functions of AGPs remain unclear.